---
layout: example.html
title: Tracing around a polygon
shortdesc: Example of setting up a draw interaction to easily snap to an existing feature.
docs: >
  This example showcases how the draw interaction API can be set up to make snapping along
  an existing geometry easier while preserving topology, which is sometimes called "tracing".
  When the user clicks on two different points on the Idaho state border,
  the part of the border comprised between these two points is added to
  the currently drawn feature.
  This leverages the `appendCoordinates` method of the `ol/interaction/Draw` interaction.
tags: "draw, trace, snap, vector, topology"
---
<div id="map" class="map"></div>
<form class="form-inline">
  <label>Geometry type &nbsp;</label>
  <select id="type">
    <option value="Polygon">Polygon</option>
    <option value="LineString">LineString</option>
    <option value="None">None</option>
  </select>
</form>
